Beginning around the time of the industrial revolution, a time when industrial process facilities were cropping up one after another, unhealthy levels of hazardous pollutants began being discharged into the environment by these industrial processing facilities. As the number of industrial processing facilities has continued to grow over the years, particularly within small geographic pockets around the country, the problem only worsened. Particularly, the discharge of pollutant gases into the atmosphere, has been linked with numerous diseases and has allegedly contributed to the formation of ozone and smog.
Since the time of the industrial revolution, as the deterioration of the environment due to process emissions became more evident and public concern became greater, the government has implemented legislation at the federal, state, and local levels in order to both monitor and reduce the number of pollutants released into the environment. Most notably, in 1970, the Environmental Protection Agency (EPA) was formed by President Nixon and significant amendments were made to the Clean Air Act. Since the Clean Air Act Amendments of 1970, numerous other amendments have been made to the Clean Air Act in efforts to further air quality standards.
As a result, virtually all large scale industrial processes in the United States are required by law to monitor their emissions on a continuous basis to ensure their compliance with emission levels of particular pollutants as set by a local regulatory agency, typically an arm of a state's EPA. This often requires the process facility to perform continuous monitoring of their emissions to avoid substantial frees and/or criminal prosecution. In order to perform these compliance measurements, many continuous emissions monitors (CEMs) have been developed to analyze the contents of the emissions of an industrial process facility.
Typically, CEMs comprise laboratory-type instruments which use analytical methods to determine the concentration of particular pollutants contained in a gas sample extracted from an emission stream of a process. These instruments are relatively expensive and complex, but are utilized because they are capable of providing the high level of accuracy required by the EPA. These instruments use a variety of known sensing methodologies to sense a target gas. For instance, non-dispersive infrared (NDIR) analyzers are used to measure carbon monoxide (CO). As another example, chemiluminescence analyzers are used to measure nitrogen oxides (i.e., NO or NO.sub.2, referred to generically hereinafter as NO.sub.x. However, because these instruments operate at room temperature and, by their nature, are sensitive to external factors, they must be remotely located with respect to the exhaust stack at a remote monitoring facility. Consequently, the gas sample that is extracted from the exhaust stack is delivered to the remote monitoring facility by a sample line that typically ranges in length between 200 to 1,000 feet. Moreover, the sample line is heated in order to prevent condensation of the water vapor, (generated by the combustion process), within the gas sample because the water condensation can combine with the acid gases in the exhaust (i.e., NO.sub.x and SO.sub.x), to form corrosive acids that may damage the sample line and the monitoring instruments. Water condensation also serves to partially scrub the NO.sub.x from the sample gas causing the monitoring instruments to read NO.sub.x at a falsely low level. Alternatively, in order to reduce and/or prevent water condensation, the sample gas can be diluted with instrument quality air at the probe using a dilution probe to decrease the water vapor concentration to a level where the water remains a vapor at ambient temperatures, and thus, alleviating the need for a heated sample line.
Further, the instruments typically require a dry gas sample. Consequently, a water knock-out system (i.e., condenser) must be provided at the monitoring facility so that any moisture in the gas sample is removed before the measurements are taken. In this regard, dilution probe systems have been found to be sufficient in reducing the water vapor in the gas sample to acceptable levels. However, dilution probes are costly and dilute the target gases to levels that require instruments with even greater relative sensitivity. Lastly, the combination of the long sample lines and the water knock-out or dilution probes systems, the volume of sample gas that must be displaced (referred to as dead volume) during measurement is considerable, generally taking the instrument more than 60 seconds to respond to changing levels of the target at the exhaust stack.
Accordingly, because these instruments must be located in a remote monitoring facility, require a heated sample line and a conditioning system to remove water vapor, the installation and maintenance costs associated with such continuous emissions monitoring systems have been quite costly, typically in the range of $200,000-$500,000 for installation alone.
In addition to performing continuous emissions monitoring, many industrial processes benefit from monitoring emissions for control purposes. Particularly, in a process utilizing combustion, control over the efficiency of the combustion is of particular importance. For example, it is preferable to have as little excess air at the point of combustion so as to prevent heat transfer to the exhaust stream which results in heat loss. Thus, by measuring the amount of air (i.e., oxygen in an exhaust stream) the appropriate changes can be implemented in the process so that only a minimal amount of excess air is found in the exhaust stream.
As another example, the efficiency of NO.sub.x reduction schemes, such as steam injection, can be monitored by taking NO.sub.x measurements before and after the steam injection process. For example, the efficiency of a selective catalytic reduction (SCR) process can be improved by measuring the amount of NO.sub.x before, or upstream, the point at which the reducing gas (i.e., ammonia) is introduced so that the appropriate amount of reducing gas is provided. This prevents the introduction of excess reducing gas into the exhaust stream, resulting in the excess reducing gas being emitted into the environment which is both wasteful and may constitute a nuisance to the surrounding community.
In contrast to continuous emission monitoring, process control monitoring requires a much faster response time, typically on the order to 15-20 seconds for each gas measured. As mentioned above, CEMs typically require more than 60 seconds between reports because of the long sample lines and the nature of the instruments as described hereinbefore.
Further, process control monitoring does not require the level of accuracy that is required by the EPA in continues emissions monitoring systems. Particularly, process control monitoring does not require an absolute value of the amount of target gas in the exhaust stream, but alternatively, only tracks trending in the concentration levels of the particular gases. Consequently, process control monitoring is typically implemented using relatively inexpensive catalytic heat-flux sensors which have lower specificity than the measurement instruments associated with continuous emission monitoring systems. For instance, catalytic heat-flux sensors are only required to measure concentrations at or above levels of 100 parts per million (ppm). Whereas, continuous emission monitoring systems are typically required to measure levels below 100 ppm.
Yet another difference exists between continuous emissions monitoring and process control monitoring in the calibration scheme utilized by each. In process control monitoring where trends are monitored, calibration is performed approximately once every calendar quarter and only on the sensor devices. This calibration scheme is referred to as unit calibration. Conversely, in continuous emission monitoring where a high level of accuracy is required in the measurements, calibration is performed approximately once every day on the whole system, including the probe and sample lines. This calibration scheme is referred to as probe calibration. In probe calibration, the calibration gas is introduced to the monitoring system at the probe rather than at the sensors, as is the case with unit calibration.
Therefore, because of the differences in process control monitoring and continuous emissions monitoring, as delineated above, separate emissions monitoring systems, and particularly, separate gas sampling systems, have historically been used.
Hence, a heretofore unaddressed need exist in the industry for a gas sampling system and method that can measure emissions with accuracy, perform unit and probe calibration, and generate reports at intervals on the order of every 15-30 seconds per gas so as to be functional for operation in both process control monitoring and continuous emission monitoring.